This invention was made with government support under Contract No. F19628-90-C-0002 awarded by the Air Force.
A number of techniques are currently employed for optically imaging biological and structural samples including x-ray imaging, ultrasonic imaging, magnetic resonance imaging and various types of nuclear imaging. While these techniques provide good results in many applications, it has been known for some years that optical imaging (i.e. imaging using signals in the optical frequency range) can provide significant advantages over other techniques in a number of applications. For one thing, optical energy sources normally require less energy and power than x-ray or nuclear sources. Use of optical imaging is, therefore, more cost-effective in many applications. Second, optical energy is generally less harmful to humans or other live subjects than other available types of radiation. Optical signals also have the potential for providing excellent contrast and resolution. Another patented advantage of optical radiation is frequency diversity; different optical properties at different wavelengths providing the potential for significant contrast enhancement over single frequency illuminiation. One area where the use of optical imaging is particularly desirable is as a replacement for current mammography techniques used for the detection of breast cancer.
However, a number of problems have prevented optical imaging from realizing its potential. First, while optical signals are not substantially absorbed when passing through a specimen (x-rays, for example, being absorbed 300% more than optical signals), because of their low energy, optical signals do tend to experience substantial scattering. For most specimens, this scattering is substantial enough so that contrast and resolution can be virtually lost or at least substantially degraded even for relatively thin specimens.
Another factor is that the dispersive properties of the specimen, and in particular variations in such dispersive properties at various points in the specimen, may provide useful imaging information concerning the specimen. Thus, the complex electric susceptibility x.sub.e of the specimen may be of interest. The complex susceptibility may be written as EQU x.sub.e =x'-ix.sub.e.sup.' (1)
where the real and imaginary components characterize the dispersive and absorptive properties of the specimen or medium, respectively. In the past, measurement of x.sub.e ' has been proposed using time-of-flight measurements employing short duration pulses and high temporal resolution recording devices. These techniques require high instantaneous power and sophisticated high temporal resolution recording devices. Another proposal has been to measure the change in phase of the optical signal to determine x'. The advantage of phase measurements are low instantaneous power and ease of determining phase relative to time-of-flight measurements. However, phase measurements of the optical signal are problematic due to phase ambiguities caused by the fact that only a slight change in the dispersive properties of the medium can result in changes in excess of 2.pi. radians at the optical wavelengths. In other words, the phase changes caused in the optical signals as a result of the dispersive properties thereof are at a wavelength which is close to that of the optical signal, resulting in ambiguities in the phase measurements. As a result, such techniques have heretofore been of only academic interest.
A need therefore exists for an improved technique for performing optical imaging on a specimen, which technique enhances resolution and contrast by substantially eliminating scatter components from the output signal and which permits the complex electric susceptibility of the specimen to be determined, and in particular the dispersive properties thereof at low power and without phase ambiguities.